In a digital communications system, digital audio data and control information is transmitted in a predetermined serial transmission format such as AES-EBU or CP-340. The AES-EBU format (Audio Engineering Society/European Broadcast Union) was developed for professional digital audio and the CP-340 format was developed for both commercial and professional digital audio. Both the AES-EBU and CP-340 formats were developed for serial transmission of two channels, each having digital audio data and non-audio, or control, data from a transmitter to one or a plurality of receivers.
The AES-EBU format transmits digital audio and non-audio data in a series of frames. The digital audio and non-audio data is typically sampled periodically by a source frequency and formed into a left audio or a right audio channel of two's complement data. The left and the right channels of digital audio and non-audio data each form a subframe. The digital audio and non-audio data is transferred in a Manchester encoded format. Manchester encoding allows information transferred in the digital audio and non-audio data to be contained in a transition from a low value to a high value, or vice versa, in any one period of the source frequency. For example, if a transition from a low value to a high value occurs during a source frequency period, a logic one is transferred. Conversely, if a transition from one electrical level to another electrical level does not occur during the source frequency period, a logic zero is transferred.
Two subframes, one for left channel information and a second for right channel information, are transmitted in sequence in any one period of the source frequency. The two subframes may also be collectively referred to as a frame. In the AES-EBU format, each subframe has a length of thirty-two time slots, where each time slot corresponds to a data bit of digital audio or non-audio information. Typically, the first four bits of each subframe are preamble bits. Preamble bits are encoded to synchronize a receiver to the source frequency of the transmitter. The next twenty-four bits transfer audio data information in two's complement form. A next bit is generally referred to as a validity (V) bit. The V bit indicates if the previous audio data information was transmitted to the receiver without any errors. The V bit is a logic zero level when the audio data information is valid, and a logic one level when the audio data information was transmitted with errors. Subsequently, a next bit is the user (U) data bit. The U bit contains user data which is associated with either the left or right audio channel. A following bit is the channel status (C) bit. The C bit is used to form a group of data bits to control transmission of audio and control information. For each of the left and right audio channels, a block is formed by accessing the C bit of each of 192 successive frames. A start of the block is identified by the preamble of the subframes. The last of the thirty-two bits of a subframe is the (P) parity bit. The P bit indicates even parity of the subframe currently transmitted. Therefore, the P bit is used to easily detect transmission errors and may be used to determine channel reliability.
The CP-340 format is very similar to the AES-EBU format. However, while incorporating a structure similar to the AES-EBU format, the CP-340 format also supports transmission of digital audio data in commercial applications. For increased versatility, CP-340 has several types of status formats. The status formats include Type I for use in broadcasting studios, Type II/Form I for use in consumer applications such as compact discs and digital audio tapes, and Type II/Form II for use in prerecorded programs. Because several types of status formats are provided, a level of accuracy for the sampling frequency must also be provided for each type of status format. Two data bits must be transmitted with the digital audio and control information data bits to indicate the level of accuracy for the sampling frequency. A first level corresponds to Type I status formats which require a high level of sampling frequency accuracy. Type II/Form I may use a sampling frequency with a Level II accuracy. Level II provides the minimum conditions which should be provided to any digital audio equipment. A level III sampling frequency accuracy is used when a variable pitch shift system is used in the transmission equipment.
For more detailed information on the AES-EBU format, refer to "AES Recommended Practice for Digital Audio Engineering-Serial Transmission Format for Linearly Represented Digital Audio Data" published by the Audio Engineering Society in 1985. Similarly, for information concerning the CP-340 format, refer to "EIAJ CP-340 Digital Audio Interface" published by the Standards of Electronic Industries Association of Japan in 1987.
Both the AES-EBU and CP-340 formats are commonly used for transmitting digital audio and non-audio between a compact disc player, a digital audio tape player, an audio mixing board, studio recording equipment, and consumer musical instruments. Because of the wide applications of the AES-EBU and CP-340 formats for transmission of audio information, it is useful for a digital signal processor to also be compatible with this digital audio format. When transferring digital audio information from a transmitter, such as a compact disc player or a digital audio tape player, and a digital signal processor, the digital data is typically provided to an interface receiver where it is modified to a form in which it may be used by the digital signal processor.
In the interface receiver, audio and non-audio data is received and converted into words of digital information with typical word lengths which are a multiple of eight bits, or a byte. For example, typical word lengths may be either sixteen or twenty-four bits. The words of digital information are easily transmitted to and received by a digital storage circuit, such as a digital signal processor, when formed into one of the typical word lengths.
Generally, audio and non-audio data corresponding to a left channel is first transmitted, and audio and non-audio data corresponding to a right channel is subsequently transmitted. As previously mentioned, the audio information is usually transferred in one of the typical word lengths which may be easily transmitted to an processed by the digital storage circuit. However, the non-audio data for each channel generally consists of only four bits-the V bit, the U bit, the C bit, and the P bit. Therefore, if the non-audio data for each channel is transmitted serially in a byte format, at least four bits of information are not used during transmission of each subframe. Consequently, during transmission of a frame of information, eight bits of information are unused for each frame of digital data. Because transmission of digital audio and non-audio data typically requires a significant number of frames of digital information, the bits which are not used form a substantial portion of the transmitted data.
To compensate for lost bandwidth when only four bits of non-audio digital data for each channel are transmitted, several techniques have been developed. For example, software programs are sometimes provided to service non-audio digital information. However, software programs require a significant amount of overhead time to execute the multiple interrupts, shifts, and initiation routines necessary to service serially transmitted non-audio digital information. When transmitted serially, the non-audio digital information is typically transferred via the same hardware channel as the audio digital information. Subsequently, the audio and non-audio digital information must be separated in the digital signal processor by extra shift and mask operations which require an extensive amount of processing time. The non-audio digital information may also be serially transferred to the digital signal processor by using an interface circuit to separate the non-audio information from the audio information. The interface circuit, however, requires extensive and complex circuitry to separate the non-audio information from the audio information in a timely manner. Therefore, current implementations of an interface circuit are generally very awkward and result in higher overhead costs.
Additionally, another technique for transferring non-audio digital data transmits each of the four bits of the non-audio digital information in parallel. The four bits of the non-audio digital data may be transferred either concurrently or separately with the transfer of corresponding audio digital data. However, a user of the digital signal processor must then be able to sacrifice at least four pins for receipt and transmission of the non-audio digital information. As well, the non-audio digital information corresponding to the left and right channels is transferred to the digital signal processor at two different times. The digital signal processor is then required to provide a software program to compensate for transferring a non-audio digital value corresponding to a right audio channel at two separate times. Similarly, a hardware circuit may also be needed to allow the two non-audio digital values corresponding to the left and right audio channels, respectively, to be processed correctly when received at two separate times. Again, overhead costs and efficiency are sacrificed.
Whether the non-audio digital information is transferred serially or in parallel, a significant amount of overhead cost and processing time is expended during the transmission of digital information to a digital signal processor. Therefore, a need exists to shorten the time necessary to process non-audio digital information in any data processing system, but especially in a digital signal processor. Additionally, a need exists for easily allowing transmission of audio and non-audio digital information by either the same or different hardware channels without resulting in higher overhead costs and processing time. Parallel transmission of non-audio digital information in a single time period is also a desirable feature. As well, the digital signal processor should provide digital audio information to an external digital audio receiver, or sink, in one of the industry standard formats, AES-EBU or CP-340.